1. Field of Invention
The invention relates to, in general, a hydraulic servo-control and, in particular, such a servo-control of a servo-controlled gearbox.
2. Description of Related Art
Servo-controlled gearboxes have become increasingly widespread. They are structurally similar to a traditional manual gearbox, except that the clutch pedal and the gear-changing stick operated by the driver are replaced by corresponding electric or hydraulic servo-controls.
Using a servo-controlled manual gearbox, the driver only needs to send the order to shift to a higher gear or to a lower gear to a gearbox control unit. The gearbox control unit autonomously changes gear acting on both the engine and the servo-controls associated with the clutch and gearbox.
Generally, the gearbox servo-control is of the “hydraulic” type and acts on a gearbox control shaft for transmitting both an axial displacement (i.e., along a central axis) for selecting the gear range and a rotation about the central axis for engaging and disengaging the single gears to the control shaft. Accordingly, the gearbox servo-control includes a linear hydraulic actuator mechanically coupled to the control shaft for axially displacing the control shaft and a rotary hydraulic actuator mechanically coupled to the control shaft for rotating the control shaft.
The gearbox servo-control includes further a storing tank containing the control fluid used by the hydraulic actuators (typically, oil) at room pressure, a hydraulic accumulator containing control fluid under pressure, a motor pump that draws the control fluid from the storing tank and feeds the control fluid under pressure to the hydraulic accumulator, and a number of solenoid valves that are adapted to selectively connect the chambers of the hydraulic actuators to the storing tank and to the hydraulic accumulator.
Hydraulic accumulators of the known type include an outer housing that is internally divided into a first variable-volume chamber adapted to accommodate the control fluid and a second variable-volume chamber adapted to receive a gas under pressure (typically, N2).
The division between the first and second chambers is typically carried out by a partition or partitions arranged inside the outer housing and axially mobile inside the outer housing between an upper “limit stop” position and a lower “limit stop” position to vary the volumes of the first and second chambers, respectively, defined inside the outer housing. In particular, the partition(s) may include, for example, a bellows element made of a metal material, a flexible partition membrane, or even a piston.
In particular, hydraulic accumulators have become increasingly applied. They are provided with an outer housing shaped as a cup-shaped body and with a piston arranged inside the outer housing and made axially sliding and mobile inside the outer housing.
The above-described hydraulic accumulators are simple and inexpensive to be implemented. However, they have the drawback that, if traumatic and large damage occurs downstream of the hydraulic accumulator (e.g., damage of about 10 mm to piping, resulting in considerable leakage of control fluid), a depression is generated such that the partition(s) is/are removed from the dedicated seat of the outer housing and damage to the whole hydraulic servo-control is caused.
Thus, an object of the invention is to provide a hydraulic servo-control of a servo-controlled gearbox. More specifically, the object of the invention is to provide such a servo-control that overcomes the drawbacks of the related art while being simple and cost-effective to be implemented.